Conventional light-pen devices are designed to operate in close proximity to video display screens. The conventional light-pen is a passive (i.e. non-light-emitting) device. It receives the light radiated from a small patch of the display screen--i.e. the light from the area of the screen the pen is directly over--and includes circuitry for generating an electrical signal only when that light is present.
Typically, the display screen is a raster scanned display device so that only a small portion of the display screen emits light at any given time. In this case, a video information generator (e.g. a host computer's video display controller) connected to the display device has knowledge of the particular portion of the screen being displayed at any time. Thus, circuitry associated with the display can use the light-pen's electrical output signal to determine which part of the display the light-pen is presently being held over. The light-pen/display controller circuitry can then act on this positional information, for example, to display a cursor at the current pen location. Alternatively, the positional information can be used by a host computer (i.e. the host computer generating the video image) to cause screen displayed objects such as windows to react.
The conventional light-pen device described above is suitable for operation in close proximity to small scale raster scanned display screens. However, the above-described conventional light-pen device is not suitable for use with large-screen projected video images, whereby a user would point at the screen from large distances using a light-pen device.
There are a number of significant problems which prevent the use of conventional light-pen devices at substantial distances from large screen projected images. Such large screen projected images are generally formed by projection devices which project light onto the rear surface of a translucent screen. The image is observed by viewing the front surface of the translucent screen. In order to prevent pointing ambiguity, a conventional light-pen device should receive light from only a small patch of the viewing screen. Since the light from the screen surface spreads isotropically, a telescopic arrangement would be required to collect the light from a small screen patch for use by a conventional light-pen device located at a large distance from the screen. The light that would be collected by such a telescopic arrangement is only the small fraction of the total light which is launched in the direction of the light-pen device. Since the amount of screen light from a projected image is small, the sensitivity of a conventional light-pen for use at large distances from a large screen projected image display would generally be inadequate. Furthermore, a light-pen device sensitive enough to respond to a small patch of screen light emitted several feet away may be easily interfered with by normal room light or other light interference. For these reasons, conventional light-pen devices are not suitable for use with large screen projected image displays and especially with large screen, rear projected video image displays.
In view of the foregoing, it is an object of the present invention to provide a light-pen system which is suitable for use with large screen projected image displays and especially large screen, rear projected video image displays.